Semiconductor laser producing light at
two wavelengths simultaneously

ABSTRACT

A SEMICONDUCTOR LASER COMPRISING ADJACENT REGIONS OF GALLIUM-ALUMINUM ARSENIDE HAVING DIFFERENT ALUMINUM CONCENTRATIONS AND DIFFERENT CONDUCTIVITY TYPES. A P-N JUNCTION IS SITUATED IN ONE OF THE REGIONS CLOSELY ADJACENT THE INTERFACE BETWEEN THE REGIONS. THE PORTION OF THE LASER BETWEEN (I) THE P-N JUNCTION AND (II) THE MECHANICAL INTERFACE BETWEEN THE REGIONS HAS A THICKNESS LESS THAN A VALUE ON THE ORDER OF TWICE THE DIFFUSION LENGTH FOR MINORITY CARRIERS IN THE SEMICONDUCTOR MATERIAL.

July 3, RE ET AL Re. 27,694

SEMICONDUCTOR LASER PRODUCING LIGHT AT TWO WAVELENGTHS SIMULTANEOUSLYOriginal Filed June 10. 196B A m m 4/4., M aM 4m MM 4 4M i 10 4 1 M P7#11T/ IT L flr FA MP: 4 m I w 1 Q H 4 4 F) fl g Nor-fork A T TOINIYUnited States Patent 27,694 SEMICONDUCTOR LASER PRODUCING LIGHT AT TWOWAVELENGTHS SIMULTANEOUSLY Henry Kressel, Elizabeth, and Frank Z.Hawrylo, Mercerville, N.J., assignors to RCA Corporation Original No.3,537,029, dated Oct. 27, 1970, Ser. No. 735,641, June 10, 1968.Application for reissue Mar. 15, 1971, Ser. No. 124,207

Int. Cl. H01s 3/00 U.S. Cl. 331-945 7 Claims Matter enclosed in heavybrackets appears in the original patent but forms no part of thisreissue specification; matter printed in italics indicates the additionsmade by reissue.

ABSTRACT OF THE DISCLOSURE A semiconductor laser comprising adjacentregions of gallium-aluminum arsenide having different aluminumconcentrations and different conductivity types. A P-N junction issituated in one of the regions closely adjacent the interface betweenthe regions. The portion of the laser between (i) the P-N junction and(ii) the mechanical interface between the regions has a thickness lessthan a value on the order of twice the diffusion length for minoritycarriers in the semiconductor material.

BACKGROUND OF THE INVENTION This invention relates to the field of lightemitting semiconductor devices, and processes for manufacturing thesame.

In the manufacture of light emitting semiconductor devices in general,and injection lasers in particular, much effect has been devoted to themanufacture of semiconductor diodes capable of emitting visible coherentor incoherent light. To this end, light emitting P-N junctions have beenformed in gallium phosphide and gallium arsenide-phosphide by vapordeposition techniques. These techniques, however, have proven difiicultto carry out in practice, and have not permitted accurate control ofimpurity concentrations and gradients in the vicinity of the P-Njunctions.

Since aluminum arsenide has a relatively high energy gap (2.2 ev.)comparable to that of gallium phosphide, some consideration might begiven to the manufacture of light emitting semiconductor devicesemploying this material. However, the hydrophilic nature of thismaterial results in deterioration when exposed to atmospheric conditionsat room temperature.

A more suitable material for the manufacture of semiconductor diodesemitting visible light is gallium-aluminum arsenide. Since the crystallattic constants of aluminum arsenide and gallium arsenide are quiteclose, lattice mismatch between a gallium-aluminum arsenide epitaxiallayer and a gallium arsenide substrate is relatively small so thatsolution growth techniques, well developed in conjunction with galliumarsenide technology, may be employed for the growth of gallium-aluminumarsenide epitaxial layers on a gallium arsenide substrate.

Prior attempts to grow gallium-aluminum arsenide epitaxial layers ongallium arsenide substrates consisted of forming P-N junctions by asolution growth process in which zinc (an acceptor impurity) was addedto a tellurium (donor impurity) doped gallium melt while the solutiongrowth process was taking place. This technique was found to possess anumber of severe drawbacks, in that the melts employed could not bereused, planar P-N junctions were ditficult to obtain, and it wasextreme- ]y dlfilCUllZ to control the impurity profile of the resultantgraded P-N junction. An abrupt P-N junction could not practically beobtained by this technique.

ice

Accordingly, an object of the present invention is to provide animproved solution growth process for forming a light emittingsemiconductor device comprising galliumaluminum arsenide.

.Another object of the invention is to provide a galliumaluminumarsenide light emitting semiconductor device capable of emitting lightat two different wavelengths simultaneously.

Another object of the invention is to provide a light emittingsemiconductor device capable of generating coherent radiation in the farinfrared range.

SUMMARY OF THE INVENTION A [multiple wavelength] semiconductor lightemitter having first and second contiguous semiconductor regions ofmutually different conductivity types. The contiguous regions form alight emitting P-N junction at the interface therebetween. The firstsemiconductor region has a thin layer of semiconductor material of agiven energy gap adjacent the interface. The energy gap of this thinlayer is substantially different from that of the remainder of the firstregion. The thin layer has a thickness less than a value on the order oftwice the diffusion length for minority carriers in the layer. The thinlayer and the remainder of the first region are capable of emittinglight at mutually different wavelengths upon recombination of minoritycarriers therein.

In the drawings:

FIG. I shows a cross-sectional view of a light emitting semiconductordevice according to the invention, at an intermediate stage ofmanufacture;

FIG. 2 shows the light emitting device of FIG. 1 upon completion of themanufacturing process;

FIG. 3 shows a light emitting semiconductor device according to analternative embodiment of the invention, at an intermediate stage ofmanufacture;

FIG. 4 shows the light emitting device of FIG. 3 after completion of themanufacturing process;

FIG. 5 shows a light emitting semiconductor device according to stillanother embodiment of the invention, at an intermediate stage ofmanufacture; and

FIG. 6 shows a light emitting device according to FIG. 5 aftercompletion of the manufacturing process.

found it desirable to prepare the initial melt with an aluminumconcentration such that x does not exceed a value on the order of 0.5.Larger aluminum concentrations than this value result in epitaxiallayers which are hydrophilic and tend to decompose at room temperature.

It is believed that when x exceeds a value on the order of 0.4, thegallium-aluminum arsenide material exhibits an indirect opticaltransistion, whereas the material is direct for values of x notexceeding this value. Since efficient light emission is presentlyobtainable only in direct materials, the aluminum concentration shouldpreferably be such that the value of x does not exceed 0.4 in any of thegallium-aluminum arsenide light emitting epitaxial layers.

A diode 1 exhibiting light emission at infrared as well as visiblewavelengths is shown in FIGS. 1 and 2. The diode 1 comprises acrystalline P type gallium arsenide substrate 2 having an N typeepitaxial gallium-aluminum arsenide layer 3 on one surface thereof. Themechanical interface between the epitaxial layer and the substrate isdenoted by the line 4. Acceptor impurities have been diffused, by heattreatment, from the gallium arsenide substrate 2 into the adjacentportion of the gallium-aluminum arsenide epitaxial layer 3 to form a P-Njunction 5 within the gallium-aluminum arsenide material.

The portion of the epitaxial layer 3 disposed between the P-N junction 5and the mechanical interface 4 defines a thin layer of relatively highresistivity (in comparison with the resistivity of the substrate 2) Ptype galliumaluminum arsenide material 6, which typically has athickness on the order of 2 microns. The thickness of the thin layer 6is substantially less than about twice the diffusion length for minoritycarriers in the epitaxial layer 3, which diffusion length is on theorder of 1 micron at 77 K., and 2 to 3 microns at 300 K. The diffusionlength is temperature dependent, as is well known in the art.

The completed diode 1, as shown in FIG. 2, comprises a pair ofelectrodes in ohmic contact with the gallium arsenide substrate 2 andthe gallium aluminum arsenide epitaxial layer 3 respectively. Theelectrode 7 in contact with the P type gallium arsenide substrate 2comprises an electroless nickel layer 8 and an overlying electrolessgold layer 9. Similarly, the electrode 10 in contact with the N typegallium-aluminum arsenide layer 3 comprises an evaporated thin layer 11and overlying electroless nickel and gold layers 12 and 13 respectively.

Upon application of a forward bias voltage to the PN junction 5 via theelectrodes 7 and 10, electrons are injected from the N type region 3into the P type thin layer 6 as well as into the P type substrate 2.Since the thickness of the layer 6 is less than a value on the order oftwice the minority carrier diffusion length, substantial electron(minority carrier) injection into the gallium arsenide substrate 2 takesplace.

The injected electrons undergo radiative recombination in the layer 6and the substrate 2, with the net result that infrared light is emittedfrom the portion of the substrate 2 adjacent the P-N junction 5, whilevisible light (due to the higher energy gap of gallium-aluminumarsenide) is emitted from the thin layer 6. Both the infrared and thevisible light are emitted simultaneously.

Some optical pumping also occurs, i.e. relatively high energy photonsgenerated in the thin gallium-aluminum arsenide layer 6 penetrate intothe adjacent portion of the gallium arsenide substrate 2 to impartenergy to electrons therein, so that these electrons are pumped acrossthe energy gap of the gallium arsenide material. This optical pumpingeffect increases the infrared light output of the diode 1, and reducesthe lasing threshold (for infrared light) when the diode 1 is employedas a laser.

Laser action is obtainable from the diode 1 by application of suflicientvoltage between the electrodes 7 and 10 to produce a current density inexcess of the lasing threshold value. The end surfaces 14 and 15 of thediode 1 are made optically flat by cleaving or polishing to form anoptical cavity. The end surfaces 14 and 15 are substantially normal tothe planar P-N junction 5. The end surface 14 is preferably made totallyreflecting by means well known in the art. while the end surface 15 ismade partially reflecting, so that light is emitted from the surface 15in the direction indicated by the arrow in FIG. 2.

Tests made on the diode 1. with a zinc doped (5 l0 /cc.) galliumarsenide substrate and a tellurium doped (3X l0 /cc.) gallium-aluminumarsenide epitaxial layer at room temperature, yielded coherent infraredradiation and noncoherent visible light at a current density on theorder of 100,000 amp/cm? At 77 K., coherent light was generatedsimultaneously at infrared and visible wavelengths, under pulsedoperation. The visible wavelength at 77 K. was 7,290 Angstroms with acorresponding threshold current density of 3,000 amp/cm.-, while theinfrared wavelength was 8,450 Angstroms, with a corresponding thresholdcurrent density of 9,400 amp/ cm. Continuous wave visible laseroperation at 6,870 Angstroms was observed at 27 K., with a correspondingthreshold current density on the order of 1,000 amp/cnt Since the twooptical beams are in the same cavity, mixing of these two wavelengthsmay occur, with the resultant shorter wavelengths, corresponding to thesum of the (mutually coherent) individual optical frequencies, beingalmost completely absorbed by the semiconductor material. The longerwavelength, corresponding to the difference between these (mutuallycoherent) optical frequencies, however, is radiated from the diode. Thislonger wavelength, which is coherent, lies in the far infrared range andhas a value on the order of 10 microns at 77 K. for the diode describedabove.

Where semiconductor emission of light (the term light is intended toinclude the near and far infrared and ultraviolet as well as the visiblerange) at two wavelengths in the visible range is desired, a structurecomprising a pair of contiguous epitaxial gallium-aluminum arsenidelayers having different aluminum concentrations (to provide differentenergy gap values) may be employed. Such structures are shown in FIGS. 3to 6.

The completed diode 20 shown in FIG. 4 is similar to that of FIG. 6,with the exception that the aluminum concentration in the P typeepitaxial layer is greatest in the vicinity of the PN junction for thediode shown in FIG. 6; in the diode 20 of FIG. 4, the aluminumconcentration in the P type epitaxial layer is lowest in the vicinity ofthe P-N junction.

The diode 20, shown in partially completed form in FIG. 3 and incomplete form in FIG. 4, comprises a P type epitaxial layer 21 of GaAlAs and a contiguous N type epitaxial layer 22 of GaAl As. Acceptorimpurities have been diffused from. the P type layer 21 into theadjacent portion of the N type layer 22 to form a P-N junction 23 spacedfrom the mechanical interface 24 between the layers. The portions of thelayer 22 disposed between junction 23 and interface 24 comprise a thinhigh resistivity (in comparison with the resistivity of the P type layer21) P type, i.e. a P- layer 25 having a thickness on the order of 2microns, which is less than twice the diffusion length for minoritycarriers in the semiconductor material.

While we prefer to main the aluminum concentration in the portion of theP type layer 21 adjacent the interface 24 substantially greater than thealuminum concentration of the thin layer 25, the diode 20 may beconstructed with equal aluminum concentrations (11:0) in the adjacentsemiconductor regions, or with an aluminum concentration in the region25 greater than that in the portion of the layer 21 adjacent theinterface 24.

The higher the aluminum concentration in the galliumaluminum aresnidematerial, the greater the energy gap. Semiconductor material of greaterenergy gap tends to transmit rather than absorb light of wavelengthcorresponding to a smaller energy gap. Since the diode 20 generateslight in the thin layer 25 and in the portion of the layer 21 adjacentthe interface 24. it is desirable that the layer 21 have a higher energygap, i.e. a higher aluminum concentration, than the layer 22 so that thelight generated in the thin layer 25 may be emitted from thesemiconductor material with minimal absorption.

Another consequence of providing a higher aluminum concentration in thelayer 21 is that the portion of the layer 21 adjacent the interface 24emits light of shorter wavelength than that emitted by the thin layer25.

The diode 20 is provided with ohmic electrodes 26 and 27 to the P and Ntype epitaxial layers 21 and 22, respectively. The electrodes 26 and 27have a construction similar to that of the corresponding electrodes 7and 10 of the diode 1.

Upon application of a voltage between the electrodes 26 and 27 toforward bias the P-N junction 23, electrons are injected from the N typelayer 22 into the P type layers 25 and 21, where they recombine to emitvisible light of two different wavelengths, corresponding to the energygaps (more precisely, to the energy level differences associated withdirect optical transitions) of the associated regions.

The diode 20 is provided with totally reflective end surface 28 and apartially reflective end surface 29, so that light is emitted from thesurface 29 in the direction shown by the arrow in FIG. 4. The reflectivesurfaces 28 and 29, as in the case of the corresponding surfaces 14 and15 of the diode 1, for an optical cavity.

Upon application of a voltage between the electrodes 26 and 27 ofsufiicient magnitude to produce a current density in excess of one orboth of the lasing thresholds (for the layers 25 and 21 respectively),lasing action occurs with consequent emission of coherent light from thesurface 29. Continuous wave operation is achieved at 8040 Angstroms at27 K., with a corresponding threshold current density on the order of420 amp/cm At 77 K., and at a current density on the order of 70,000 to90,000 amp/cm. simultaneous lasing at two different wavelengths in thevisible range may be obtained from the diode 20, under pulse operation.Electrical and optical coupling between these two wavelengths rendersthem mutually coherent, so that optical mixing to provide a longerwavelength (corresponding to the difference between the individualoptical frequencies) may take place in the optically nonlinearsemiconductor material.

In the diodes shown in FIGS. 2, 4 and 6, the relative intensities andwavelengths of the two optical outputs generated may be varied bychanging the operating temperature of the diode. Such a temperaturechange varies the diffusion length for minority carriers and thereforeaffects the proportion of the injected electrons which recombine in eachof the two (P and P type) radiating regions.

The light emitting diode 40 shown in FIGS. 5 and 6 is, as previouslymentioned, similar to the diode shown in FIGS. 3 and 4, differing onlyin the aluminum concentration profile within the P type region. Thediode 40 has adjacent P and N type gallium-aluminum arsenide epitaxiallayers 41 and 42, respectively. Acceptor impurities have been diffusedfrom the P type region 41 into the adjacent portion of the N type region42 to form a P-N junction 43 therein.

The P-N junction 43 is spaced from the mechanical interface 44 betweenthe layers 41 and 42, so that the portion of the layer 42 disposedbetween junction 43 and interface 44 comprises a thin layer 45 of Pconductivity type having a thickness (on the order of 2 microns) lessthan a value on the order of twice the diffusion length for minoritycarriers in the semiconductor material. The diode 40 is provided withelectrodes 46 and 47 similar to the electrodes 26 and 27 of the diode20, respectively.

The diode 40 functions in similar fashion to the diode 20, but, due toits higher aluminum concentration in the portion of the P type layer 41adjacent the interface 44, operates at shorter wavelengths and withimproved efliciency in comparison with the diode 20.

The manufacture of the diode 1 is commenced by preparing the P typemonocrystalline gallium arsenide substrate 2 with a clean major surfaceoriented parallel to the l00 or 1l1 crystallographic plane. Thesubstrate 2 is initially heavily doped with zinc to a concentration onthe order of 5 l0 /cm.

The substrate 2 is then placed in a suitable boat with the cleaned majorsurface thereof exposed. The wafer 2 is held at one end of the boat bymeans of a suitable clamp, and the boat is oriented so that the endcontaining the substrate 2 is tilted upward. A melt comprising gramsgallium, 5 grams gallium arsenide, 5 milligrams tellurium and milligramsaluminum is disposed at the lower end of the boat. The melt is heated toa temperature on the order of 920 C., at which time the boat is tippedso that the cleaned surface of the substrate 2 is exposed to the melt.

If desired, selenium or tin, rather than tellurium, may be employed asthe donor impurity material.

The melt is then allowed to cool to a temperature on the order of 400C., at which time the boat is tilted to remove the melt from the surfaceof the substrate 2, and the substrate is removed from the furnace.During the time the melt cools in the presence of the substrate 2, agallium-aluminum arsenide alloy is precipitated therefrom to form anepitaxial layer on the substrate 2, the furnace cools from 920 C. to 400C. in approximately 25 minutes, most of the epitaxial solution growthtaking place between the 920 C. temperature and a temperature on theorder of 750 C.

The preferred aluminum concentration in the melt may vary from 1 to 250milligrams (other quantities of melt ingredients than those specifiedmay be employed so long as the proper relative proportions aremaintained); we have found that aluminum concentrations in excess of 250milligrams result in epitaxial deposition of a transparent layer whichapparently comprises nearly pure aluminum arsenide.

As previously discussed, aluminum arsenide is unsuitable for practicallight emitting diodes, and it is therefore desirable to keep thealuminum concentration below the 250 milligram value. We prefer tomaintain the aluminum concentration below 150 milligrams, correspondingto concentrations yielding direct semiconductor material, i.e. materialin which optical transitions do not require phonon assistance.

The foregoing process results in an N type epitaxial layer 3 which has athickness in the range of 1 to 3 mils, and which is doped with telluriumto a concentration on the order of 3 l0 /cm. In order to realize asharply graded P-N junction which lies within the gallium-aluminumarsenide layer 3, the resultant structure is heat treated at atemperature on the order of 900 C. for a time on the order of 30minutes. During the heat treatment, zinc diffuses from the substrate 2into the adjacent portion of the epitaxial layer 3 to form a P-Njunction 5 which is situated 1 to 2 microns from the mechanicalinterface 4 between the epitaxial layer 3 and the gallium arsenidesubstrate 2.

This heat treatment step may also be carried out simultaneously with thesolution growth of the epitaxial layer 3, merely by holding thetemperature of the melt at about 900 C. for a period on the order of 15to 30 minutes while the layer is being grown, and then allowing the meltto cool as previously described.

After the epitaxial layer 3 has been grown, the exposed surface thereofis cleaned, lapped, polished and etched in accordance with techniqueswell known in the gallium arsenide technology art. The substrate 2,which initially has a thickness on the order of 18 mils, is lapped sothat the completed diode 1 has a total thickness on the order of 4 mils.Preferably, the 4 mil final thickness should be equally divided betweenthe remaining portion of the substrate 2 and the epitaxial layer 3.

After lapping of the substrate 2 and cleaning, polishing and etchingthereof in accordance with prior art techniques, the electrodes 4 and 7are deposited on the exposed major surfaces of the P and N type regions.First a thin tin layer 11 is vacuum evaporated onto the exposed surfaceof the N type layer 3, while maintaining this layer at a temperature onthe order of 550 C.

Both surfaces of the wafer are then immersed in (i) an electrolessnickel bath followed by (ii) an electroless gold bath to provide thefinal electrode structure shown in FIG. 2.

The diodes shown in FIGS. 3 and 4 are manufactured by successive growthof P and N type gallium-aluminum arsenide epitaxial layers onto amonocrystalline gallium arsenide substrate 50 having a major surfaceoriented parallel to the or ll1 crystallographic plane. The substrate 50may be of either conductivity type, or may be substantially intrinsic.

A gallium-aluminum arsenide P type epitaxial layer 21 is deposited, bythe solution growth process, onto a cleaned major surface of thesubstrate 50.

The P type epitaxial layer 21 is formed by precipitation from a meltcomprising 25 grams gallium, grams gallium arsenide, 5 grams zinc and 50milligrams aluminum.

The resultant melt is heated to a temperature on the order of 920 C. andapplied to the exposed surface of the gallium arsenide substrate 50. Themelt is then allowed to cool to 400 C. over approximately a 25 minuteperiod, at which time the melt is removed from the exposed surface ofthe substrate 50, and the substrate 50 is removed from the furnace.

The resultant gallium arsenide epitaxial layer 21 is of P typeconductivity with a zinc impurity concentration on the order of 5X l0/cm.

The thickness of the epitaxial layer 21 is on the order of 1 to 3 mils.

After suitably cleaning the exposed surface of the epitaxial layer 21, atellurium doped N type epitaxial layer 22 having a thickness on theorder of 1 to 3 mils is deposited by the solution growth process ontothe exposed surface of the layer 21. The N type epitaxial layer 22 is Ldeposited in the same manner as that employed for deposition of the Ntype epitaxial layer 3 in conjunction with the manufacture of the diode1.

The resultant structure is heat treated at 900 C. for (i) to 30 minutesduring the solution growth of the epitaxial layer 22, or (ii) 30 minutesafter the solution growth of the epitaxial layer 22, to form the P-Njunction 23.

The gallium arsenide substrate 50 is next removed by lapping, and ohmicelectrodes 26 and 27 are applied to the epitaxial regions 21 and 22,respectively. The electrodes 26 and 27 are formed by methods similar tothose employed for formation of the corresponding electrodes 7 and 10 ofthe diode 1.

Where the substrate 50 is of relatively low resistivity P conductivitytype, it may be lapped to a suitable thickness and employed to provideohmic contact to the adjacent epitaxial layer 21.

The diode 40, shown in FIGS. 5 and 6, is fabricated by similar processesto those previously described. The P type epitaxial layer 41 is grown ona gallium arsenide substrate 51 which is similar to the substrate 50.The solution growth process employed for deposition of the P typegallium-aluminum arsenide epitaxial layer 41 is substantially identicalto that employed for deposition of the P type layer 21 of the diode 20.

Due to the solubility vs. temperature characteristics of aluminum in thesolution growth melt, the highest concentration of aluminum occursinitially, i.e. at the mechanical interface 44 between the P typeepitaxial layer 41 and the gallium arsenide substrate 51 (see FIG. 5).

The next step in manufacturing the diode 40 is the removal of thesubstrate 51 by lapping, and the cleaning, etching and polishing of theunderlying surface of the epitaxial layer 41 (adjacent the interface 44)therebu exposed.

The epitaxial layer 41 is then inverted and placed in the solutiongrowth boat. An N type epitaxial layer 42 is solution grown, by themethod previously described for growth of the epitaxial layer 3 of thediode 1, onto the newly exposed P type epitaxial layer 41, so that theinterface 44 between the epitaxial layers 41 and 42 is adjacent theportion of the P type layer 41 which has the highest aluminumconcentration.

The resultant structure is then cleaned, lapped, polished and etched inaccordance with prior art techniques, and ohmic electrodes 46 and 47 areapplied to the P and N type epitaxial regions 41 and 42, respectively.The electrodes 46 and 47 are formed according to processes similar tothose applied for formation of the electrodes 7 and 4 of the diode 1,respectively.

While the manufacturing processes which have been described above formaking the diodes and 41 involve deposition of a P type gallium-aluminumarsenide layer on a gallium arsenide substrate, followed by depositionof an N type gallium-aluminum arsenide layer on the P type layer, theorder of deposition of the epitaxial layers may be reversed, otherprocess steps remain the same.

For example, the diode 20 may be prepared by growing an N type epitaxiallayer on the gallium arsenide substrate 50, growing a P type layer onthe N type layer, and heat treating the resultant structure to driveacceptor impurities from the P type layer into the N type layer to forma P-N junction therein. The gallium arsenide substrate may then beremoved or, where the substrate comprises relatively low resistivity Ntype material, it may be lapped to a suitable thickness and employed toprovide ohmic contact to the adjacent N type layer. Similarly, the diode40 may be manufactured by depositing an N type gallium-aluminum arsenideepitaxial layer on the gallium arsenide substrate 51, removing thesubstrate 51, and depositing a P type epitaxial layer on the surface ofthe N type layer which was previously contiguous with the substrate 51.Heat treatment may then be employed, as before, to drive acceptorimpurities from the P type layer into the adjacent N type to form a P-Njunction therein.

We claim: 1. A semiconductor light emitter, comprising: first and secondcontiguous semiconductor regions of mutually different conductivity typeforming a light emitting P-N junction at the interface therebetween,

said first region having a thin layer of semiconductor material of givenenergy gap adjacent said P-N junctron,

the energy gap of said layer being substantially different from theenergy gap of the remainder of said first region, said thin layer havinga thickness less than a value on the order of twice the diffusion lengthfor minority carriers in said layer,

said thin layer and the remainder of said first region being capable ofemitting light of mutually different wavelengths upon recombination ofminority carriers therein.

2. A light emitter according to claim 1, said emitter generatingcoherent light at at least one of said wavelengths, further comprisingan optical cavity having refiective surfaces substantially normal tosaid P-N junction, said junction being substantially planar.

[3. A multiple wavelength light emitter according to claim 2, whereinsaid emitter generates coherent light at each of said mutually differentwavelengths, further comprising optically nonlinear means for mixingsaid mutually different wavelengths to produce coherent radiation at agiven wavelength different from either of said mutually differentwavelengths.

4. A multiple wavelength light emitter according to claim 3, wherein thesemiconductor material of said emitter serves as said opticallynonlinear means.

5. A multiple wavelength light emitter according to claim 1, whereinsaid thin layer comprises gallium-aluminum arsenide] 6. A light emitteraccording to claim [5] 10, wherein said second region comprisesgallium-aluminum arsenide.

7. A light emitter according to claim 6, wherein said remainder of saidfirst region comprises gallium-aluminum arsenide or gallium arsenide.

8. A light emitter according to claim [5] 10 wherein said thin layercomprises gallium-aluminum arsenide having the formula Ga Al As, where xdoes not exceed a value on the order of 0.4.

9. A light emitter according to claim 10 wherein said remainder of saidfirst region comprises gallium-aluminum arsenide and has an energy gapgreater than that of said thin layer.

10. A semiconductor light emitter, comprising:

first and second contiguous semiconductor regions of mutually dlfierenrconductivity type forming a light emitting P-N junction at the interfacetherebetween, said first region having a thin layer of gallium-111wminum arsenide semiconductor material of given en- 9 energy gap adjacentsaid P-N junction, the energy gap of said layer being substantiallydifferent from the energy gap of the remainder of said first region,said thin layer having a thickness less than a value on the order oftwice the a'iflusion length for minority carriers in said layer, saidthin layer and the remainder of said first region being capable ofemitting light of mutually different wavelengths upon recombination ofminority carriers therein.

References Cited The following references, cited by the Examiner, are ofrecord in patented file of this patent or the original patent.

UNITED STATES PATENTS 3,245,002 4/1966 Hall 331-945 3,300,671 1/1967Woodbury 313-108 3,309,553 3/1967 Kroemer 313l08 3,387,163 6/1968Queisser 313-108 3,412,344 11/1968 Pankove 331--94.5 3,456,209 7/1969Diemer 331-94.5 3,501,679 3/1970 Yonezu et al 317-234 OTHER REFERENCESAlferov et al.: High-Voltage P-N Junctions in Ga,,Al ,,As Crystals,Chemical Abstracts, vol. 69, 1968, abstract No. 62840g.

Rupprecht et al.: Eflicicnt Visible Electroluminescence at 300 K. FromGa ,,,Al As P-N Junction Grown by Liquid-Phase Epitaxy," Applied PhysicsLetters, vol. 11, pp. 81-83, Aug. 1, 1967.

Susaki: Lasing Action in (Ga, Al )As Diodes, IEEE Journal of QuantumElectronics, vol. QE-4, pp. 422-424, June 1968.

Rupprecht et al.: Stimulated Emission From Ga Al,,As Diodes at 77 K.,IEEE Jour. of Quant. Elect, vol. QE-4, January 1968.

Parrish et al.: A Technique for the Preparation of Low-Threshold RoomTemperature GaAs Laser Diode Structures," IEEE Jour. of Quant. Elect,vol. QE-S, p. 210.

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EDWARD S. BAUER, Primary Examiner U.S. Cl. X.R. 3l7--234 R

